Many diversified applications have been found for titanium and its alloys. Titanium metal has been essential to the aerospace industry since the early 1950's because it combines a high-strength to weight ratio with the ability to perform at much higher temperatures than aluminum or magnesium. It has therefore been used in compressor blades, turbine disks, and many other forged parts of jet engines and aircraft frames. It is also widely employed in the chemical processing industry because of its excellent resistance to chloride corrosion. Because of its scarcity and high cost, titanium has frequently been used in the form of a titanium powder to produce articles which are too expensive or difficult to produce by machining or forging from massive metal shapes. More efficient processes for the production of titanium powder have therefore been sought.
A majority of the world's titanium is made by the Kroll process, which produces titanium "sponge" in the form of a metallic powder. The titanium sponge is produced by reducing titanium tetrachloride (TiCl.sub.4) with magnesium or sodium in a heated steel vessel. After cooling, an intimate mixture of titanium sponge and frozen chloride salt forms. The sponge and salt are separated by crushing and water leaching the products to dissolve the salt and produce a purer titanium product. The titanium sponge is then compressed into an electrode bar and vacuum arc remelted (VAR) to consolidate the metallic sponge. The expensive VAR process must be repeated once and sometimes twice to remove residual chloride salt and produce a clean consolidated bar of titanium. Alloying agents may be introduced during resulting if special purpose titanium alloys are desired.
The most important consideration for any process of making titanium is to prevent contamination with either metallic or non-metallic impurities, because even small amounts of some impurities can make the product brittle and unworkable. This is an especially serious problem for aerospace and other critical applications where such impurities can lead to defects in the final product manufactured from titanium. It is crucial, for example, that titanium components of jet engines or guided missiles maintain their structural integrity at all times in stressful environments. To help preserve this integrity, many processes have been developed for producing titanium powder free of contaminants which impair the structural integrity of the end product.
U.S. Pat. No. 4,602,947, for example, discloses a method of producing titanium sponge or titanium alloy powder by reducing gaseous titanium tetrachloride with magnesium. This method, which is schematically summarized in FIG. 2, produces titanium metal in the form of finely divided particles by first forming a liquid mixture of titanium and zinc, then solidifying the liquid mixture to produce finely divided alloy particles, and finally evaporatively separating zinc from the particles to produce pure titanium powder. In particularly disclosed embodiments, titanium chloride vapor is injected into a molten zinc-magnesium bath. Titanium replaces magnesium in the liquid alloy such that liquid zinc titanium and liquid magnesium chloride are produced. The less dense liquid magnesium chloride, which is completely immiscible with the liquid zinc titanium alloy, floats to the top of the reactor where it is removed. The resulting liquid zinc titanium mixture is recovered, solidified, and passed to a zinc evaporation zone where the zinc is sublimed to produce sponge titanium.
Although the process disclosed in U.S. Pat. No. 4,602,947 produces a relatively pure titanium sponge product, it suffers from the expensive drawback of requiring large amounts of zinc. Titanium has a very low solubility in zinc at temperatures up to the normal boiling point of zinc (907.degree. C.). As a practical matter, the titanium solubility in liquid zinc is limited to about five weight percent. This is shown by the zinc rich end of the zinc titanium binary phase diagram reproduced in FIG. 1. This low solubility is significant because the solubility limit cannot be exceeded if a liquid mixture of titanium and zinc is desired. Such a liquid mixture is required in the '947 patent, and because of the limited titanium solubility, approximately 20 lbs. of zinc must be consumed for each pound of titanium produced. A substantial amount of zinc is also lost through evaporation at the elevated temperatures preferred in that prior process. Although a cover of molten salt theoretically prevents zinc evaporation up to its boiling point at the gas over-pressure (usually one atmosphere or less), as a practical matter it is usually necessary to operate at temperatures over 907.degree. C. to increase the solubility of titanium in zinc. The zinc evaporates at this temperature and is lost from the reaction.
Other United States patents disclose methods for producing titanium sponge by reducing titanium chloride salts with aluminum. See, for example, U.S. Pat. Nos. 4,359,449; U.S. Pat. No. 4,390,365; and U.S. Pat. No. 4,468,248. None of these patents disclose reduction of gaseous titanium chloride by magnesium in a liquid zinc alloy. Other U.S. patents teach producing titanium powder and titanium alloy powder from binary and more complex zinc-titanium alloys by removing the zinc through sublimation. Such patents include U.S. Pat. No. 4,470,847; U.S. Pat. No. 4,595,413; and U.S. Pat. No. 4,655,825. Removal of zinc from zinc titanium alloys is also taught in U.S. Pat. No. 4,602,947.